In the last video we saw that
the two chair shapes, or the two chair configurations
of just plain vanilla cyclohexane, were
equally stable. There was nothing that would
imply that this is more stable than that or vice versa. What we want to do in this video
is address what happens if, instead of just a pure
cyclohexane, what if we added a methyl group to it? And so instead of just a
cyclohexane, let's think about a methylcyclohexane, so we know
what to do when we're naming things. So methylcyclohexane will
look like that. We'll have a hexagon here. This is the way we've been
drawing it historically. We've been drawing it
just like that. And then we'll have
a methyl group. And you could literally just
draw a methyl group like this or maybe like this. And by implication, you have a
carbon here, essentially a CH3 group here, and, of course,
you have another hydrogen bonded here. Now, if you were to draw this
methyl group in three dimensions like this, it would
look just like our different chair positions before, but
instead of this H being just an H, we could make this entire
methyl group, so we could turn this CH3 right here,
so this-- let me draw it out in a different color. We could make this carbon that
implicitly has three hydrogens on it, so this is a CH3 group
that is a methyl group that's right there. We could substitute one of these
hydrogens with it, and then this would be one
of the shapes of this methylcyclohexane, CH3. Let me write it down, just
to practice our naming. So this is methylcyclohexane. Now, this is one of their chair
positions, and then if this were to flip down and the
other side were to flip up, the other chair position would
take this methyl group from being in an axial position,
and it would put it in an equatorial position. So this is the same
group right here. Let me put a circle around it. It's a whole group. That's the CH3. Now, the question we want to
answer in this video, is one of these two configurations
going to be more stable? Now, you might just be able to
eyeball it looking at this diagram, and say, hey, when
we're in the position where the methyl group is in the axial
position, it's going to be closer to all of these
other carbons over here, closer to their electron
clouds. Maybe it has higher
potential energy. Maybe it'll want
to spring away. And if that was what you're
guessing or what you're kind of eyeballing, you'd be right,
because in this position, this methyl is much further away
from all of the stuff out here, so there'll be less, I
guess you can consider it, electron cloud crowding,
if you will. Now, to see that a little bit
more clear, I want to do what we call a double
Newman diagram. And really, that's the whole
motivation of this video is to kind of expose you to that. A Newman diagram isn't just
useful for simple things like a butane or an ethane. You can actually do it
with cyclical rings. And to do the Newman diagram,
let me number these carbons. So we could number
them like this. So this is carbon one, two,
three, four, five, six. And you don't have to call this
one methylcyclohexane, because whenever you have only
one group attached to the ring, you implicitly start
numbering at the carbon that the group is attached. So that is the one
carbon over here. This is the one carbon, two,
three, four, five, six. And over here, once again, is
the one carbon, two, three, four, five, six. Now, what I want to do is draw
two Newman projections, and both of them will involve--
well, actually, I'll draw four, but you'll see what I'm
talking about in a second. So the first Newman projection,
I'm going to start at that carbon right
over there. So let's think about what
that would look like. So we're at the front. We're staring straight
on to that carbon. So this carbon's in the front,
this carbon over here. This carbon, carbon number
two, is in the back. So let me label this. This is carbon number one that
we're looking head on. Now, right in the axial
position you have that methyl group. So let me draw that. So in the axial position you
have that methyl group right over there. And then in the equatorial
position right here, you have this hydrogen. Let me draw that. So you have this hydrogen. And then over here, this bond
right over here, this gets us to another CH2 group. Let me draw that like this. So that gets us to
another CH2. That's a CH2 right
there, right? Or maybe I should circle
it like this. This whole thing is a CH2. So it's bonded to the carbon,
but the carbon has two hydrogens on it. And this is actually
carbon number six. So this is the front. Now, if we're staring straight
and carbon number two is right behind it-- let me draw
carbon number two. I'll do it in this blue color. So carbon number two,
that bigger circle. Now carbon number two, what's
going on over there? It has a hydrogen in the axial
position going straight down. This hydrogen right
over there. And it has another hydrogen
going equatorial. So this green hydrogen right
here is going equatorial. It looks like that. And then it connects to
carbon number three, which is another CH2. It has two hydrogens branching
off of it. In front, we have carbon
number one. In the back, we have
carbon number two. Let me color code it
a little better. Carbon number two. And then carbon number two
branches off to-- so if you think of this branch right here,
that's carbon number two branching off to carbon number
three, or CH3 right there. Let me do that in a new color. So this is actually
a CH2 group. That's a CH2. This is a carbon, and it has
two hydrogens, right? So this is a CH2. This is carbon number
three right there. And then that goes-- and
actually, I'll pause there. What I'll do now is I'll draw
another Newman projection, but for this Newman projection,
we'll be looking straight on our carbon number five. We're going to see we're going
to form a ring because the first Newman projection
we just did essentially covers this bond. This bond is sitting straight
into the screen the way I did it right now. Carbon number two is directly
behind carbon number one. So I guess the opposite side of
the ring is five to four. If you look at, here, one to two
is there, and then five to four is just like that. So you can imagine when we're
doing the Newman projections, we're looking straight on here
on the left Newman projection. We're going to look straight in
that direction on the right Newman projection. So if we have carbon number five
in front, what are its bonds going to look like? Well, it's going to bond to
carbon number six over here. So we can make this right--
let me do that in a different color. This right here, this bond right
here is this bond right here to that same CH2 that
our first Newman projection bonded to. And then he's going to
have two hydrogens. I haven't drawn them here. Let me draw them, just
so you can see them. It's going to have two
hydrogens, one in an axial position and one in an
equatorial position. It's hard to see now, but he's
going to have one hydrogen in an axial and one in an
equatorial position. And then in the back
we're going to draw carbon number four. And carbon number four, I will
do in this green color. So carbon number four--
actually, , well, yeah I'll do carbon number four in
that green color. This is really just an exercise
in visualization. That's why I wanted
to do it with you. So carbon number four has an
axial hydrogen, so it has a hydrogen pointing
straight down. That hydrogen is
that hydrogen. It has an equatorial hydrogen
going out like that. And then it bonds to carbon
number three. So this bond right here is
this bond just like that. So what do we see immediately
when we draw this chair position? When our methyl group
is in the axial position, what do we see? Methyl in axial position. We see that it's gauche,
or gowche. I don't know the best
way to pronounce it. It's only 60 degrees away from
carbon number three. This is only 60 degrees,
or a dihedral angle. When you use the Newman
projection, this is only a 60-degree angle. It is gauche to carbon
number three. So maybe they're crowding
each other a little bit. Let's compare it to the
situation where our methyl group is equatorial, where the
carbon that it's attached to is on the down part
of the chair. Let's see what that Newman
projection looks like. So same thing. Let me scroll over to the
right a little bit. So this is this configuration
and it's in equilibrium with this configuration right here. We'll do the exact
same exercise. Carbon number one. But now carbon number one, in
carbon number one right here, the hydrogen is now axial and
it's pointing straight down. Let me draw that. So we have a hydrogen pointing
straight down now. Hydrogen's pointing down and
now the CH3 is in an equatorial position, which you
can see more clearly on this than over here. So you have a CH3. The methyl group
is right there. And now this bond to
carbon number six will look like this. So you have a CH2. This is number six
right over there. And now if we were to go to
the back, if we were go to carbon number two in the back,
which we had done in the blue color before, so I'll do it
in the blue color again. Carbon number two in black, it
has a hydrogen in the axial position going straight up. That's that hydrogen
right there. It has a hydrogen going
straight up. It has another hydrogen over
here, and then it bonds to carbon number three in the
back, so CH2 over here. So this guy's the same thing
as this guy, but now we've flipped configurations, so he
bonds to that guy in the back. And now we do the Newman diagram
looking straight on to carbon number five, or looking,
actually, straight on to-- right, carbon
number five. So we're looking straight in
this direction for this Newman projection. Now we're going to look straight
in this direction for our other Newman projection. So carbon number five, if we
draw it in the front-- I haven't drawn it here, but it
has a hydrogen at the axial position, and it has
a hydrogen in the equatorial position. So carbon number five, if we
look at this, and this is really-- if you're getting a
little stressed out about this because it's a little hard
to understand, you might want to rewatch. This is really just an example
of visualizations. I really hope this isn't
confusing you. If you find this really
daunting, this isn't going to really trip you up in the rest
of organic chemistry, but if you can get it, it's
even better. You'll be that much better
at visualizing some of these molecules. So if we look straight on at
carbon number five, we have a hydrogen in the axial position
coming straight down. It is bonded. Carbon number five is bonded to
carbon number six, which is right over there. So let me make it very clear. So this bond right here,
I want to do it in a different color. This bond it right here is the
same thing as this one. This is a really good way to--
if you can do this, then your brain is pretty good at
translating between Newman projections and these kind
of seat diagrams that we have up here. So this is this bond. This axial hydrogen is
this axial hydrogen. And then we have another
hydrogen. We have this hydrogen right
here, which would be that like that. And then behind it, you have
carbon number four. Carbon number four is like that,
so carbon number four, draw a circle. It has a hydrogen in its axial
position, another hydrogen like that, and then it bonds
to carbon number three. So this was number
three right here. So just like that. So what do we see about this
methyl group here? In this situation, this
methyl group is anti-. There's two ways to
think about it. It's dihedral angle versus
carbon number three is now 180 degrees. Over here it was gauche. Its dihedral angle to carbon
number three was 60 degrees. Now, it's 180 degrees. So it's much further. And now its dihedral angle
to carbon number six is also 120 degrees. So in this situation, where
our methyl group is equatorial, it's axial here. It's equatorial here when we
jumped back down because notice, it's parallel. The bond is parallel to
parts of the ring. In this situation, we are
farther away from the other methyl groups. There's less crowding,
and so this is a more stable situation. So you could say it is
anti-configuration relative to carbon number three, while over
here, it was gauche to carbon number three. I don't know if I'm saying it. Is it gowche, gauche? I don't know the best
way to pronounce it. So in this case, there's
less crowding. This is more stable, lower
potential energy. So hopefully you found
that interesting. This was really a way, just an
exercise in being able to go from this visualization
to kind of this double Newman diagram. And if it makes it any easier,
the way you could think about it is we're kind of viewing in
this Newman diagram right here, where this carbon number
six is this carbon number six. This CH2 is this
one back there. So when we're looking at it
from this, we're kind of looking at this hexane ring
from this direction. We see this in front. This is this, and we
see that in back. That is that. Over here, same thing. We're looking at it directly
from this direction. We see this guy on top is over
here, and this guy on the bottom is over here.